ggplot2: Import transparent-background PNG logo into R

Having trouble inserting PNG logo from desktop and having it integrate with the ggplot chart below. I read there were issues using raster with on Windows platform but can't be sure if that's the case.

   library(png)
    library(grid)
    library(ggplot2) 
    library(reshape2)

    img <- readPNG(system.file("img", "Rlogo.png", package="png"))
    g <- rasterGrob(img, interpolate=TRUE)

    df <- melt(economics[, c("date", "psavert", "uempmed")], id="date")

    ggplot(df, aes(x = date, y = value)) +
          geom_line(aes(color = variable), size=1) +
          scale_color_manual(values = c("#999999", "#E69F00")) +
          theme_minimal() +
          annotation_custom(g, xmin=as.numeric(as.Date("1980-01-01")), xmax=as.numeric(as.Date("1990-01-01")), ymin=-Inf, ymax=5)

Also, I imported my logo but the image is either just a black box or a white box and not displaying the logo. How can I resolve this issue? I checked to ensure the logo.PNG wasn't encrypted or protected. Thank you for your time.